My buddy started up a Truck with 18 year old gas, an old 61 GM Truck, put in a batt and slapped on a carb 6 cranks and it fired up. I have started a Subaru with 5 year old gas in it. I know that supposedly it can go bad, but I sure havent had a problem yet.
Actually, gas only lasts about 3 months before it starts forming deposits on everything it touches. The problem is worse in hotter climates because the lighter elements in the fuel evaporate and leave the heavier elements in the tank causing the fuels volatility to get very low.
Just drive over to the gas station and fill up as soon as possible. It will probably not run quite right until you have run a couple tanks of gas through it.
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There isn't a specific point in time, like after 152 days, gas is bad.
Also, forming deposits/varnish is something else that happens, but not whats menat by gas going bad or stale.
Gas goes 'bad' when the more volatile compounds vaporize off.
See, fuel is blended to have a certain vapor pressure(measured in Reid Vapor Pressure, or RVP), that is a measure of how easily the gasoline will vaporize.
This is VERY important to the function of a car.
If you are in a hot climate, and/or a hot engine compartment, and the gas is too volatile, then it will vaporize in the lines, causing vapor lock. This isn't nearly as much of a problem, now that fuel pumps are pushing the fuel, instead of sucking the fuel like older mechanical pumps.
So, you want to make sure the fuel isn't too volatile, right?
Well, if its cold outside, and your engine is cold, then a fuel with low volatility will hit the cold intake manifold and condense into liquid fuel on the runners. And you don't start.
Here in michigan, it is common to have RVP of about 15 in the winter, and RVP of 9 or so in the summer.
If you filled your tank with summer gas, it might not run in the winter, and vice versa. I have a lawnmower very succeptable to this.
Going back to your question, as fuel gets older, especially if its in a warm area, the vapor pressure of the gas contantly gets lower, as the lighter compounds vaporize off. Your gas filler cap may be sealed, but you also have an evaporative emissions system, with a charcoal canister, and a purge valve. The gas tank IS vented, it leads tot he charcoal canister, which absorbs all the vaporized fuel, then sucks it back into the engine when the purse solenoid is activated.
So, the answer is totally: it depends..
You'd probably be able to run just fine with old gas, most cars compensate well. But you also might need to run a few things of fuel system cleaner through it all to help remove deposits.
(btw, sorry for the long response.. I'm bored at work, and used to work as a fuel system engineer working on evaporative emissions, so that was my cup of tea..)
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you can leave gas in there for a long time and it will work but it wont have the performace of new gas, like for example, say you fill your tank with 92 then let it sit for 4 years, dont put a motor in there and then boost it to 10psi and think its 92 octane so its all good. it loses its kick
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